In communication systems operating over a bandwidth, the transmission quality depends over the frequency and over the time. Given a system with a sufficiently wide bandwidth, different frequencies within the system may provide different capacity between particular nodes of the system, at any one time. Typically frequencies close together will have similar capacity, and those spaced further apart may have different capabilities.
Ideally, channel knowledge (e.g. obtained from measuring the reception of known reference signals) is used to identify the better frequencies on which to communicate between a given set of nodes at a given time. However this creates a number of system overheads in terms of transmission of reference signals and exchange of measurement information between nodes.
In some cases, transmissions should be scheduled without knowledge of which frequencies will give the better performance. In these instances, it can be advantageous to make use of frequency diversity, that is, scheduling the transmissions across a number of different frequencies, where the chosen frequencies for a given allocation are somewhat separated. In some cases there may be limited information on channel performance, which is sufficient to allow choice of transmission formats (e.g. modulation and coding schemes) to be used for transmissions on particular frequencies, but which is not sufficient to make choices on which frequencies should be scheduled to which allocations.
Such scheduling of allocations may be fixed, or may be adaptive. In either case, the receiving node needs to have knowledge of the scheduling being used for its transmissions at any given time, in order to be able to successfully receive and decode the transmission.
In order to achieve best system performance, a distributed scheduling allocation needs to optimise against two different criteria. One is the frequency separation achieved within any allocation, which should be as large as possible, given the available resource units to be able to test over a large band of frequency. The other criterion is the amount of signalling required to enable the receiving node to identify the location of the transmissions is it intended to receive, which should be minimised.
A fixed scheduling has the advantage that minimal extra signalling is required. In fixed scheduling, a plurality of sets of resources blocks are predetermined, and the only signalling required for each node is an indication of which set of resource blocks has been allocated to this node. The main disadvantage of fixed scheduling is that the frequency separation also must be fixed and cannot be optimised according to the system load. Typically, the separation is set at half the maximum possible separation—which provides optimal separation under fully loaded conditions, but suboptimal separation under other conditions.
A known variation of fixed scheduling is mirrored allocations, where the available lower frequency resource block is paired with the available higher frequency resource block, the second lower frequency resource block with the second higher frequency resource block etc. However, as the loading increases, allocations come closer to the middle of the frequency band, ultimately with the neighbouring frequencies in the middle of the band being scheduled together as a single allocation. This negates the benefit of frequency diversity in these middle regions, and additionally provides a different separation for each allocation, leading to inconsistent performance across allocations.
On the contrary, free scheduling can provide optimal frequency separation. In this case, there are no pre-constraints on the identification of allocations, so these can be made with total freedom at the transmitting node. However, in order for the receiving node to operate, it is necessary to signal the details of the components of every allocation to the receiving node. In the case where comparatively small amounts of useful data are being scheduled, the overheads due to this signalling may remove any system benefits due to the improved radio performance.